This invention relates to mail transmission technology with which it is possible to send an encrypted mail using an encryption method compatible with the mail user agent device likely to be used to receive the mail.
In an S/MIME-encrypted mail, the message body is encrypted with symmetric key encryption. There are many types of symmetric key encryption, and these are variously supported by different mailers (mail user agent devices). When the same mail address is used on different mailers, for example one mailer being used in a home and others outside the home, because of differences in the symmetric key encryption methods supported by the mailers, the situation may arise that mail can be read by one mailer and not by another. Thus it is desirable for S/MIME-encrypted mail to be sent which can be received by any mailer among a plurality of mailers from which the recipient uses the same mail address.
And in a mailing list server (PKI-ML server) using public key infrastructure, when an S/MIME message which has been hybrid-encrypted using a public key of a mailing list is received, a re-encrypted S/MIME message is sent out to all the members of the mailing list. Here, the encryption algorithms and key lengths of public key encryption used by members can be determined by means of members' certificates. However, the symmetric key encryption capabilities of mailers used by different members are diverse. Consequently, when the same symmetric key encryption method is always used with respect to all members, there is the possibility that there will be some members who cannot read an encrypted message.
Thus it is desirable for attributes of encryption methods including symmetric key encryption methods to be managed and for messages to be encrypted by encryption methods compatible with the mailers used by members.